This invention is directed to novel vanilloid receptor VR1 ligands. More particularly, this invention relates to novel hetero isonipecotic amides that are potent modulators of VR1.
Noxious chemical, thermal and mechanical stimuli excite peripheral nerve endings of small diameter sensory neurons (nociceptors) in sensory ganglia (e.g., dorsal root, nodose and trigeminal ganglia) and initiate signals that are perceived as pain. These neurons are crucial for the detection of harmful or potentially harmful stimuli (heat) and tissue damage (local tissue acidosis and/or stretch) that arise from changes in the extracellular space during inflammatory or ischaemic conditions (Wall, P. D., and Melzack, R., Textbook of Pain, 1994, New York: Churchill Livingstone). Nociceptors transduce noxious stimuli into membrane depolarization that triggers action potential, conducts the action potential from the sensory sites to the synapses in the CNS, and conversion of action potentials invokes a perception of pain, discomfort, and appropriate mechanical/physical protective reflexes. At the molecular level, nociception is carried out by ion channels or receptors. Plant derived vanilloid compounds (capsaicin and its ultrapotent analog, resiniferatoxin, etc.) are known to selectively depolarize nociceptors and elicit sensations of burning pain—the sensation that is typically obtained by hot chili peppers. Therefore, capsaicin mimics the action of physiological/endogenous stimuli that activates the “nociceptive pathway”. Recent advances in pain biology have identified receptors for vanilloids, protons (i.e., acidic solutions), and for heat. Because nociceptors are involved with unwanted pain and inflammatory conditions in human beings and animals, modulation of their nociceptive pathway is important in palliative and other therapies.
Walpole and colleagues at Sandoz reported on the first competitive antagonist of the sensory neuron excitants capsaicin and resineriferatoxin (Walpole, C. S. J. et. al., J. Med. Chem. 1994, 37, 1942). Subsequently, capsazepine has been shown to be a vanilloid receptor antagonist.